The invention relates to a process for testing closed containers, e.g. the residual air volume or the quality and tightness of closures which are attached to containers, mechanical oscillations being excited in the closure, the mechanical oscillations being analyzed, measurement values being derived therefrom, these measurement values being compared with pre-set values for acceptable closures and a signal being produced which indicates whether the measurement value corresponds to a value for an acceptable closure.
It is known from DE-A-40 04 965 to test negative-pressure resilient-cap closures, in particular negative-pressure screw-closures, for the tightness of their attachment by producing mechanical oscillations in the top, processing and evaluating the oscillations with regard to their frequency, their period duration and/or attenuation and determining therefrom the level of the negative pressure in the container.
A similar process for measuring the internal pressure of a closed container is known from U.S. Pat. No. 5,353,631, a wall of the container being impacted, the oscillation spectrum of the mechanical oscillations produced thereby being recorded and being compared with the previously recorded oscillation spectrum of containers with known internal pressures.
In practice, problems arise from the fact that a drinks-filling operation obtains the blanks for screw closures or crown-cap closures from various manufacturers and the blanks of individual manufacturers differ from each other in material composition or thickness or in the compound attached to the inside of the closure. Such differences can also occur between different batches from the same manufacturer. As a result of these deviations, a particular measurement frequency for closures from one manufacturer can lie in an acceptable range, but lie in an unacceptable range for those of a different manufacturer, so that often no reliable statement about a closure is possible.
The object of the invention is to improve the reliability of the process named at the outset for testing container closures.
This object is achieved according to the invention in that the mechanical oscillation properties of the closures are ascertained additionally before their attachment to the containers and in that the acceptable range of values which are ascertained after the attachment of the closures to the containers is chosen according to the oscillation properties which were ascertained before the attachment of the closures.
By means of the first test of the closures which takes place before the attachment of the closures, which are still closure blanks, the material thickness can for example be ascertained by frequency measurement or the thickness of the compound layer by measuring the attenuation. It has been shown that closures which are manufactured with blanks which have the same material thickness, material composition, thickness of compound etc. deliver measurement values in the second test which scatter in only a very narrow range when the closure sits correctly, the container internal pressure, the fill level height etc. are correct. Difficulties which arise in practice from the fact that the closure blanks differ from each other can be dealt with by initially examining the properties of the closure blanks in a first test which takes place before the attachment of the closures to the containers. It is sufficient for this purpose to ascertain the oscillation properties of the blanks, e.g. the frequency of the mechanical oscillation and its attenuation. Using value tables in which the oscillation features ascertained before and after the attachment of the closures are allocated to each other, a very narrow range of acceptable measurement values can then be specified for a greater number of closures which differ from each other.
In principle, two versions of the process according to the invention are possible:
In the first version, it is assumed that a limited number, e.g. four, different types of closure blanks are involved. The first test of the closures is therefore carried out only with aim of ascertaining the type of the respective closure concerned. In the second test, carried out after the attachment of the closure, it is then ascertained whether the closure concerned has oscillation values within the range of acceptable values valid for this type.
In the case of the second version, oscillation features determined during the first test of the closure blank, e.g. the natural frequency and the time integral of the amplitude, are measured. For the second measurement, the acceptable values or value ranges are then chosen according to the values measured during the first test, e.g. a frequency increase between 10 and 20% or frequency shift of 500 Hz and a 30% reduction in the time integral of the amplitude. A previously empirically determined correlation is produced between the values of the first measurement and those of the second measurement.
Both versions can also be combined in such a way that, during the first test, the closure blanks are distinguished according to different types and then a determined correlation between the measurement values of the first and the second test is then applied for each type.
The process according to the invention is particularly suitable for refining the process described in the earlier application DE 197 36 869.7, in particular for determining the air volume and thus the residual oxygen in the drinks bottles. This test is important above all for beer. In the case of the process described in the earlier application DE 197 36 869.7, the analysis of the mechanical oscillations is carried out directly after the attachment of the closure before a significant change in the internal pressure takes place in the container. This oscillation analysis is the second test in the case of the process according to the invention.
The process according to the invention can also be used however for test processes in which the mechanical oscillations are analyzed and evaluated only after a certain period of time after the attachment of the closure. A precondition for this is that the path of a closure and bottle to which it is attached can be tracked. For this, there are known processes as used to track a bottle from an inspection device to a diversion device.
In particular when testing a closure for tight fit, it is expedient to carry out the second test or an additional test at some time after the attachment of the closure in order to be able to ascertain a drop in pressure that has occurred in the meantime. It may e.g. be necessary to measure the internal pressure before and after pasteurization of the container""s contents. In a pasteurization apparatus, there are a very large number of containers in unordered sequence. To still be able to identify the containers, these must be marked. For this, the closures or the closed bottles must be provided with a marking in print or in bar code. This marking may be visible or for example detectable only under UV light. A magnetic marking as described in the simultaneously filed international patent application xe2x80x9cVerfahren zum Prxc3xcfen von Behxc3xa4lterverschlxc3xcssenxe2x80x9d [Process for testing container closures] (=DE patent application 198 34 185.7) is particularly suitable.
The process according to the invention is suitable both for crown caps which are crimped and for screw closures the screw thread of which is first shaped by rolling, and for twist-off closures and for can tops. Devices for attaching closures to drinks bottles, so-called closers, consist in general of several closing organs, e.g. crimping or rolling organs. It was ascertained that the individual closing organs differ slightly from each other in their mode of operation and produce closures with different values for relaxation time, energy and the frequency of the oscillation pattern, as is described in the earlier application DE 197 36 869 mentioned above. Preferably, individual value tables or correlation values are therefore used for each closing organ.